The background of the invention will be set forth in two parts.
1. Field of the Invention
This invention relates to delay lines, and more particularly to bulk acoustic wave delay lines.
2. Description of the Prior Art
The usefulness of propagating elastic wave energy in solids has been known for many years. Utilizing this technology, such devices which store and delay signals have been developed to a relatively high degree. Many texts are presently available which thoroughly describe the history and advancements of this art, such as, for example, "Ultrasonic Methods in Solid State Physics" Rohn Truell, Charles Elbaum and Bruce B. Chick, Academic Press, 1969.
Probably the greatest interest in the field of bulk wave devices has been in bulk acoustic wave delay lines. Unlike surface acoustic wave delay lines in which most of the energy propagating along an elastic surface is converted to electromagnetic wave energy upon reaching a state of the art transducer, only about 10% of the propagating bulk wave energy is converted at an output transducer, the rest being reflected back toward the input transducer. This relatively strong reflected wave is again reflected at the input transducer and is incident on the output transducer to produce a relatively strong signal known generally as the triple-transit signal.
Although there was at first much interest in bulk acoustic wave devices because they are more adaptable for operation in the multi-gigahertz range as compared to surface acoustic wave devices (usually limited to about 500 MHz), the problem of the triple-transit signal has caused a decrease in such interest.
In attempts to overcome spurious multiple transit signal problems resulting from reflections from the crystal end faces, it has been found that these unwanted signals are attenuated or suppressed through careful design utilizing several effects:
A. Attenuation -- if the main signal is attenuated .alpha..tau. dB, then the triple transit signal is attenuated an additional 2.alpha..tau. dB.
B. Diffraction loss -- due to spreading.
C. Tilting the end faces of the crystal to cause phase cancellation and beam walk-off.
D. Acoustic matching of the transducer in order to reduce the acoustic reflection.
Generally, all of these effects are utilized to some extent in order to obtain what has been considered to be a reasonable value of triple-transit suppression of 30 dB, where triple-transit suppression is defined as the ratio of the main delayed signal to the triple-transit spurious signal. It has now been determined that while diffraction loss in the main signal can be relatively large (20 to 30 dB, or more), the additional diffraction loss in the triple-transit signal, and hence the triple-transit suppression, is theoretically limited to 9.5 dB. This limit has been described in several articles, including one entitled "Today's Microwave Acoustic (Bulk Wave) Delay Line", in Microwave Journal, 13, March 1970, pp. 67-76, by Frank A. Olson; and an article by E. K. Sittig, "High Speed Ultrasonic Digital Delay Line Design", in Proc. IEEE, 56, July 1968, pp. 1194-1202.